This application claims the priority of patent application Ser. No. 951 07 891.4 filed in the European Patent Office on May 24, 1995, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a bearing arrangement for a pump shaft for delivering newtonian or non-newtonian liquids of different viscosities, having a bearing for the shaft, a gear pump, and to a process for collecting and returning leakage liquid passing through a bearing during the pumping of liquids of different viscosities.
In the case of pumps, such as gear pumps, it cannot be prevented or it is not necessary to prevent that a small portion of the liquid medium to be delivered, which is relatively highly pressurized, is driven through a slide bearing, for example, along the drive shaft. As a rule, this leakage flow is used for lubricating the slide bearing. After passing through the slide bearing, the leakage liquid is collected in a collection groove or in a relief groove in order to be returned by way of the connection duct of the pump to its inlet side. Such sealing and returning arrangements are known, for example, from German Patent Documents DE-544 963 and 31 35 037. As long as liquids with largely constant viscosities are continuously delivered in a pump, the known return arrangements are perfectly sufficient. In contrast, during the discontinuous pumping of liquids having very different viscosities, the described arrangements are unsuitable.
Thus, for example, during the manufacturing of polyester, a discontinuous or batch-type polymerization can be carried out for process-related reasons. A gear pump can be used for this purpose. During the polymerization phase or circulation phase, very low-viscosity liquid first arrives in the pump causing an extremely low pressure difference in the pump. After the conclusion of the polymerization phase, that is, during the extrusion phase, the pump delivers high-viscosity to medium-viscosity molten mass against pressure consumers connected behind it, such as filters, extrusion tools and the like, causing the pump to have to deliver against a clearly higher pressure difference.
With respect to the pump, the following difficulty will arise:
During the delivery of the highly viscous product, that is, during the extrusion phase, the pressure difference above the pump is sufficiently high for causing a satisfactory lubricating flow or leakage flow through the bearings as well as through the return duct system of the suction side of the pump. After the change-over to the circulation operation, that is, during the pump operation in the polymerization phase, the differential pressure is very low although it is sufficient for the low-viscosity fluid medium. However, at the point in time of the change-over, high-viscosity liquid is still situated in the return system and after the change-over, because of the low differential pressure, can be replaced only very slowly by the new low-viscosity liquid. Because of a stagnation of the bearing leakage current, there is the potential danger of an undersupply of the bearing which may result in the risk of damage to the bearing.
It is therefore an object of the present invention to suggest a return arrangement on a pump shaft which ensures a sufficient leakage lubrication during the delivery of liquids of different viscosities. In particular, it is an object of the invention to ensure the leakage lubrication when a change-over takes place from the delivery of a high-viscosity medium to the delivery of a low-viscosity medium; that is, an extremely impaired flow or even a clogging of the return arrangement for the low-viscosity liquid is prevented.
The above object is achieved by means of a bearing arrangement or a return arrangement for a pump shaft of a pump for the delivery of liquids of very different viscosities having a bearing for the shaft wherein at least one collection or relief zone is arranged on a low-pressure side of the bearing for collecting leakage liquid padding through the bearing along the shaft, and wherein at least two return ducts of bores are provided which connect the collection or relief zone with a suction side of the pump in order to return the collected leakage liquid.
The return duct system suggested according to the invention of a pump for the delivery of liquids of different viscosities (for example, ##EQU1## has at least one collection or relief zone arranged either on each back side of the bearing or on both pump lids, for collecting leakage liquid which penetrates through the bearing along the shaft and which is simultaneously responsible for lubricating the bearing. From the collection or relief zones, the return system has at least two return ducts or bores respectively in order to return the collected leakage liquid to the suction side of the pump.
Preferably, at least one of the ducts or bores can be closed which, depending on the viscosity of the medium to be delivered in the pump, now makes it possible to close one of the bores or to keep all bores or return ducts open. When high-viscosity media are carried, the bore is to be closed for the low-viscosity medium. When the low-viscosity medium or product is delivered, the bore must be open for the low-viscosity medium. In the latter case, the flow resistance ratio is such that the return bore for the high-viscosity product appears closed to the low-viscosity product.
Additional preferred embodiments of the bearing arrangement defined according to the invention or of the return arrangement defined according to the invention are characterized in the by one or more of the following features
(i) at least one of the ducts and bores can be selectively closed; PA1 (ii) the ducts or bores are constructed to be selectively closable independently of one another; PA1 (iii) the ducts or bores have different diameters or cross-sections; PA1 (iv) the collection or relief zone is formed by at least one annulus adjoining the shaft; and PA1 (v) at least one annulus extends on the back side or low-pressure side at least partially in a slide bearing body or on or in a pump lid, and at least one of the return ducts or bores extends at least partially through or along a slide bearing housing.
In the case of a gear pump, for example, having two return ducts, the return bore which is situated closer to the back side of the bearing or the low-pressure side is reserved for the return of the low-viscosity liquid. Thus, after the change-over from the delivery of a high-viscosity medium to the delivery of the low-viscosity medium, this closer return bore is opened up, which return duct will then not be "clogged" by the high-viscosity medium. Coming back to the above-mentioned example of a discontinuous manufacturing of polyester, this closer return duct will be opened up after the change-over to the polymerization and circulation operation. As the result of the polymerization, the viscosity in the polyester to be circulated will rise and, after a certain viscosity has been reached or after a certain differential pressure has been reached or after the change back to the extrusion operation, the closer return duct is closed again so that no high-viscosity material can enter into it. The closing and opening of this closer return bore can take place hydraulically, pneumatically or by means of an electric motor, such as a step motor. Finally, whether the shorter return duct extends through the slide bearing or through the housing surrounding the slide bearing depends, for example, on the geometrical situations. However, both embodiments can be used in any case.
When several slide bearings are used which are arranged side-by-side, it is naturally possible to place return ducts of the same type together at least along a common section; that is, return ducts for low-viscosity media can be placed together as well as return ducts for high-viscosity media.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.